Free cutting alloys



Patented Jan. 7, 1933 UNITED STATES PATENT OFFICE FREE CUTTING ALLoYs Louis W. Keinpf and Walter A. Dean, Cleveland, Ohio, assignors to Aluminum Company of America, Pittsburgh,- Pa., a corporation of Pennsylvania No Drawing.

Application September 18, 1933, Serial No. 689,882

3 Claims. (01. 75-1) This invention relates to aluminum base alloys and it is particularly concerned with alloys of this nature containing substantial amounts of silicon together with other elements such as 5 copper, nickel, magnesium and the like The development of intemal combustion enmal expansion closely akin to that of cast iron, the material commonly used for cylinder blocks.

A type of aluminum base alloy of this character is one wherein a substantial amount of silicon 20 is used in conjunction with smaller quantities of other elements. While such alloys are well adapted to use in internal combustion engines, they nevertheless present a difficult problem in machining because of the trouble encountered in 25 obtaining an evenly cut, smooth surface. This condition arises in part through the occurrence of hard particles of elementary silicon distributed throughout the alloy which appear to have seg- 'gregated during cooling of the alloy from thechinability is, therefore, eminently desirable especially from the standpoint of economical production of the finished article and. from the be- I 40 havior of the part in service.

One of the objects of our invention is to diminish the irregularity in cut caused by the hard silicon particles in an alloy of the kind described in a simple, economical manner and to thereby 45 obtain a smooth, uniformly cut, machined finish. Another object is to accomplishthe fore going end without substantial detriment to the physical properties of the alloy.

We have now discovered that the addition of 5 lead, bismuth, 'cadmium, and/or thalliumgreat 1y improves-the machinability and appearance of an aluminum, base alloy' containing silicon, nickel, copper, and magnesium in spite of any hard particles that may be present. An alloy of 55. this type adapted to the manufacture of wrought or cast-pistons is one which contains from about 7 to 15 per cent silicon, 0.5 to 7 per cent nickel, 0.3 to 4 per cent copper, and 0.2 to 3 per cent magnesium. The machining quality of such an alloy may be markedly improved by the addition 5 of at least one of the class of elementslead,

bismuth, cadium, and/or thallium in amounts of from about 0.1 to 6 per cent; For thepurpose' of our invention the aforesaid elements are substantially equivalent in alloys of the type herein-1o described byreason of their similar effect upon the machining quality of said alloys.

Although lead, thallium, bismuth, and/or cadmium may be added to an aluminum-siliconnickel-magnesiumecopper alloy in the proportion hereinabove disclosed, we prefer to use between about 1 and 4 per cent for many applications. We have further found that better results are oftenobtained by adding not less than about 1.5 per cent of cadmium or bismuth to the aforementioned class of'alloys. For-many purposes a basefalloy is very satisfactory which contains from about 10m 15 per cent of silicon, from about 2 to 5 per cent of nickel, from about' 0.5 to 2 per cent of copper and from about 0.2

to 1 per cent of magnesium. Machining tests on alloys within this range containing additions of lead, thallium, bismuth, and/or cadmium have shown a marked superiority over the same alloy without the added elements.

As a particular example of the improvement in machinability obtained through the application of our invention, the case of an alloy which is employed in making wrought pistons for aircraft motors may be cited. The design of such pistons requires that considerable machining. be done to finishthem for service. An alloy used for this purpose has a nominal composition of 12.5 per cent silicon, 0.8 per cent nickel, 1.15 per cent magnesium, and 0.8 per cent copper, the balance being aluminum. Pistons made from this alloy frequentlyhave score marks on the machined surface where a particle of elementary silicon may have been' drawn across the metal by the tool in the course of the machining operation. 5

The variation in hardness of the surface, prob-'- ably caused by the presence of elementary silicon, also tends to product an irregular machined surface. It is very difficult to obtain satisfactory machined surfaces on the above alloy on a com- I mercial scale with ordinary cutting tools without a frequent resharpening. By the addition of about 3-per cent of lead to an alloy of the composition given above, it is possible to obtain a very smooth surface with an ordinary carbon steel cutting tool. The chips flow freely from the article being machined and are shorter and more breakable than when no lead is added to the alloy. There appears to be no undesirable chatter or vibration of the cutting tool which would be induced by a marked variation in hardness.

,A uniformly smooth cut is made by the tool indicating that greater cutting speeds might be used if desired with a corresponding decrease in machining cost.

The elements lead, thallium, bismuth, and cadmium are not only beneficial to the machining quality of the alloy when separately used, but they may often be more effectively employed in combination. It has been found that the simultaneous presence of two or more of the indicated elements frequently produces a degree of machinability not attained by the use of an equivalent amount of a single element. For example, an aluminum base alloy containing about 12.5 per cent of silicon, 0.8 per cent ofnickel, 1.15 per cent of magnesium, 0.8 per cent of copper, and about 1 per cent each of lead and bismuth, balance substantially aluminum, machined more readily under test than the same alloy containing only 2 per cent of lead and no bismuth. Likewise a combination of 2 per cent each of lead and bismuth produced a better machining quality in the alloy than 4 per cent of lead alone. The total amount of the added elements here referred to should not exceed about 6 percent, and preferably a maximum of 4 per cent total is adhered to The lead, thallium, bismuth and/or cadmium,

are most conveniently added to the molten alloy in solid metallic form since they melt at a temperature considerably below that of the aluminum and aluminum-silicon base alloys. If more than about 1.5 per cent or these elements is to be added to the alloy, the molten bath should be heated somewhat above the ordinary melting temperatures and vigorously stirred to assure a uniform 5 mixture. The method of adding heavy low melting point metals to aluminum or its alloys here referred to is more fully described in co-pending application Serial No. 689,885 now Patent No. 1,959,029, granted May 15, 1934, filed September 10 18, 1933. Alloys disclosed but not claimed herein are claimed in our copending applications, Serial'Nos. 19,618, 19,619, 19,620, 19,621, 19,622 and 19,623, filed May 3, 1935. When cadmium is added to the alloy, however, the temperature 15 should not exceed about 1400 F. to avoid volatilization of the metal.

- to the usual thermal treatments familiar to those skilled in the art for the purpose of improving 25 or altering their physical characteristics.

We claim:

1. An aluminum base alloy containing about 12.5 per cent of silicon, 0.8 percent of nickel, 1.15 per cent of magnesium, 0.8 per cent of copper, and 3 per cent of lead, the balance being aluminum.

2. An aluminum base alloy containing from about 7 to 15 per cent of silicon, from about 0.3 r

to 4 per cent of copper, from about 0.2 to 3 per cent of magnesium, from about 0.5 to 7 per cent of nickel, and from about 0.1 to 6 per cent of lead, the balance being aluminum.

3. An aluminum base alloy containing from about 7 to 15 per cent of silicon, from about 0.3 0 to 4 per cent of copper, from about 0.2 to 3 per cent of magnesium, from about 0.5 to 7 per cent of nickel, and from-about 1 to 4 percent of lead,

the balance being aluminum.

LOUIS w. KEMPF;

WALTER A. DEAN. 

